Fixed-wing aircraft and helicopter control surfaces such as flaps, rudders and, in the case of helicopters, blades, are typically positioned for the control of the aircraft by hydraulic actuators. Such hydraulic actuators usually include a piston connected to the control surface by a connecting rod and reciprocable within a cylinder, selective pressurization of the cylinder on opposite sides of the piston positioning the piston and thus the control surface connected thereto. It is common for a single control surface to be positioned by pairs of actuators whereby, in the event that one of the actuators is damaged, a redundant actuator may set the control surface for continued operation of the aircraft. Thus, it will be understood that if damage to one of the actuators is manifested in a jamming of the piston within the cylinder, the other actuator must not only power the control surface in the normal movement thereof, but must also overcome such jamming by movement of the piston past the point of the jam. Where in the case of combat aircraft, the jammed actuator has been damaged ballistically, such jamming is often the result of a rupture of the cylinder from the exterior thereof by a projectile such as a bullet. Such a rupture will inwardly deform a portion of the cylinder wall forming an obstruction therein to traversal of that portion of the cylinder by the piston. When the projectile strikes the connecting rod, damage to the rod may include cratering of the rod or formation of protuberances thereon which jam in the actuator cylinder gland (end portion).
In an effort to devise piston structures capable of traversing obstructions in the cylinder walls, and cylinder glands capable of allowing damaged connecting rods to pass therethrough, it has been proposed that the pistons and glands be provided with frangible portions whereby upon encountering an obstruction in the cylinder wall or connecting rod, the effected frangible portion will rupture around the obstruction thereby allowing the piston to move past the obstruction or the damaged rod to pass through the gland. Examples of such frangible pistons and glands are found in U.S. Pat. No. 3,884,127 to Simmons. In the Simmons patent, the piston and gland are provided with a plurality of circumferentially spaced segments defining a groove which receives an annular seal, the segments being spaced by radially extending slots and frangible along a weakened portion or scribed line. The frangible segments may also be provided with an annular rib which upon encountering an obstruction, provides mechanical advantage for breaking off the segments.
It will be appreciated by those skilled in the art that the provision of the scribed line, the rib, and the radially directed slots in the Simmons piston and gland necessarily make a contribution to the complexity of the overall shape of these actuator components. Additionally, the provision of the radially directed slots adds to the risk of actuator fluid leakage past the piston under conditions of normal operation.